The injection molding process requires that a molten thermoplastic material, or melt, be maintained in its molten state such that the melt may be injected into a mold cavity to be formed into various products. The heat profile of the melt must be maintained throughout the injection molding process in order for the melt to retain its desirable properties and thereby produce a satisfactory product. In general, the melt enters an injection molding or hot runner manifold via a bushing where it is transported to one or more injection molding or hot runner nozzles to be subsequently injected into the cooled mold cavity.
A manifold for use in a hot runner system must have certain properties. For example, the manifold must have good heat transfer characteristics and be able to withstand high temperatures and pressures. Consequently, a typical manifold is made from a block of metal with a high heat transfer coefficient.
If the manifold is formed from a block of metal, however, it becomes difficult to form melt channels within the manifold that possess appropriate geometries for efficiently and effectively transporting melt therethrough. Typically, the melt channels are made by boring holes from the outside of the metal block into its interior, usually by gun drilling techniques. As it is desirous to have multiple hot runner nozzles connected to a single manifold, the “bores” through the manifold that form the melt channels necessarily include at least one curve. Further, this curve must be rheologically balanced which is virtually impossible to achieve with standard gun drilling techniques. A rheologically balanced curve in the manifold melt channel is desirable or the melt cannot flow smoothly along the curve, which can result in either a turbulence in the melt, an obstruction of the flow or an accumulation of solidified melt along the walls of the manifold melt channel. Such turbulence may result in undesirable pressure changes in the injection molding system and heat exchange between the melt and manifold that can ruin the melt and the subsequent product formed therefrom. Further, nooks created during the gun drilling process create spaces in the manifold melt channels into which melt can get embedded causing another undesirable result. For example, the manifold melt channels may become difficult to clean between runs and/or melt hardened in the nooks may break-off and be present in subsequent molded product.
One method for forming a rheologically balanced curve within a block manifold is to intersect a bore from the side of the block and a bore from the bottom of the block to create substantially a 90° angle. A plug with a curved surface is then inserted via the side bore and placed within the manifold such that the plug's curved surface forms a more gradual curvature in the manifold channel. However, such plugs are expensive and time consuming to manufacture and assemble.
Another method of creating a rheologically balanced manifold melt channel has been to divide the manifold into an upper and lower portion, wherein half of the melt channel can be formed in a mating surface of each portion. Generally, the manifold is formed in an upper and lower portion wherein each portion has one half of a melt channel formed into a mating surface thereof. The upper and lower manifold portions are then bolted and sealed together to form rheologically balanced melt channels. However, this system requires the use of very accurate equipment.
Another challenge in utilizing a metal block for a hot runner manifold is that it can be difficult to uniformly heat the manifold melt channels therein. Typically, a heater is attached to the exterior of the block, and in some instances it can be brazed or pressed into a heater channel. This is a time consuming process, as it requires the machining of the heater channel which has a complicated contour.
Uniform heating of a manifold may be created by utilizing an internal heater disposed in a split-block manifold, such as that described above. In this case, the heater is placed in a special bore between the upper and lower portions of the manifold along the melt channels.
Another solution for creating a manifold with rheologically balanced melt channels and uniformly heating the melt channels includes the use of pre-formed pipes with a rheological curve. However, it is difficult to effectively heat pipes without placing them in a thermoconductive medium. Oil or gas mediums have been tried to effectively heat the melt channels. However, the safety and effectiveness of such an approach is questionable especially if oil, due to its flammability, is used as the thermoconductive medium.
Other embodiments use pipes that are either unitarily formed or cut and welded together to create smooth passages. However, seams between the two pipes may be weak and break at high temperatures and pressures and may cause the manifold to malfunction.
Thus, it is desirous to have an injection molding hot runner manifold with a manifold melt channel and integral heating system that solves the above-identified problems.